Restore the demo list position on start

It's quite anoying to have to search in the list
the last item played everytime the demo is restarted.

Fix that by saving and restoring it.

PiperOrigin-RevId: 299109261

RELEASENOTES.md

@@ -90,6 +90,7 @@
     ([#6920](https://github.com/google/ExoPlayer/issues/6920)).
 *   Metadata: Add minimal DVB Application Information Table (AIT) support
     ([#6922](https://github.com/google/ExoPlayer/pull/6922)).
+*   The demo app startup selected item is the last played one.
 
 ### 2.11.3 (2020-02-19) ###
 

demos/main/src/main/java/com/google/android/exoplayer2/demo/SampleChooserActivity.java

@@ -17,6 +17,7 @@ package com.google.android.exoplayer2.demo;
 
 import android.content.Context;
 import android.content.Intent;
+import android.content.SharedPreferences;
 import android.content.res.AssetManager;
 import android.net.Uri;
 import android.os.AsyncTask;
@@ -62,6 +63,8 @@ public class SampleChooserActivity extends AppCompatActivity
     implements DownloadTracker.Listener, OnChildClickListener {
 
   private static final String TAG = "SampleChooserActivity";
+  private static final String GROUP_POSITION_PREFERENCE_KEY = "SAMPLE_CHOOSER_GROUP_POSITION";
+  private static final String CHILD_POSITION_PREFERENCE_KEY = "SAMPLE_CHOOSER_CHILD_POSITION";
 
   private boolean useExtensionRenderers;
   private DownloadTracker downloadTracker;
@@ -69,13 +72,15 @@ public class SampleChooserActivity extends AppCompatActivity
   private MenuItem preferExtensionDecodersMenuItem;
   private MenuItem randomAbrMenuItem;
   private MenuItem tunnelingMenuItem;
+  private ExpandableListView sampleListView;
 
   @Override
   public void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.sample_chooser_activity);
     sampleAdapter = new SampleAdapter();
-    ExpandableListView sampleListView = findViewById(R.id.sample_list);
+    sampleListView = findViewById(R.id.sample_list);
+
     sampleListView.setAdapter(sampleAdapter);
     sampleListView.setOnChildClickListener(this);
 
@@ -163,11 +168,32 @@ public class SampleChooserActivity extends AppCompatActivity
           .show();
     }
     sampleAdapter.setSampleGroups(groups);
+
+    SharedPreferences preferences = getPreferences(MODE_PRIVATE);
+
+    int groupPosition = -1;
+    int childPosition = -1;
+    try {
+      groupPosition = preferences.getInt(GROUP_POSITION_PREFERENCE_KEY, /* defValue= */ -1);
+      childPosition = preferences.getInt(CHILD_POSITION_PREFERENCE_KEY, /* defValue= */ -1);
+    } catch (ClassCastException e) {
+      android.util.Log.w(TAG, "Saved position is not an int. Will not restore position.", e);
+    }
+    if (groupPosition != -1 && childPosition != -1) {
+      sampleListView.expandGroup(groupPosition); // shouldExpandGroup does not work without this.
+      sampleListView.setSelectedChild(groupPosition, childPosition, /* shouldExpandGroup= */ true);
+    }
   }
 
   @Override
   public boolean onChildClick(
       ExpandableListView parent, View view, int groupPosition, int childPosition, long id) {
+    // Save the selected item first to be able to restore it if the tested code crashes.
+    SharedPreferences.Editor prefEditor = getPreferences(MODE_PRIVATE).edit();
+    prefEditor.putInt(GROUP_POSITION_PREFERENCE_KEY, groupPosition);
+    prefEditor.putInt(CHILD_POSITION_PREFERENCE_KEY, childPosition);
+    prefEditor.apply();
+
     Sample sample = (Sample) view.getTag();
     Intent intent = new Intent(this, PlayerActivity.class);
     intent.putExtra(